1. Field of the Invention
This invention relates to input/output apparatus for interfacing field wiring with electrical control and monitoring systems. In particular, it relates to a modular system having multiple strings of modular units serially connected to a process controller with each of the modular units having multiple plug-in modules. More particularly it relates to arrangements for connecting the modular units in series and locking the multiple modules in the modular units.
2. Background Information
Electronic control systems and monitoring systems such as process control systems require communication with the real world. This includes receiving inputs such as commands and process conditions provided by various sensors, transducers and/or contacts, generating outputs such as control signals, status and results. This interaction is provided by input/output (I/O) hardware and/or firmware. The signals that pass through the I/O can be analog and/or digital in nature. Typically, the I/O interface includes signal processing circuitry, which for example can limit the range of analog signals, provide filtering, amplification or attenuation, surge protection, isolation, pulse shaping for digital signals, analog to digital conversation for inputs, digital to analog conversion for outputs, and other signal conditioning.
Conventionally, the input/output function has been performed by circuitry implemented on plug-in I/O cards mounted in racks within I/O cabinets. U.S. Pat. No. 5,472,347 suggests a modular I/O system in which a selection of I/O cards each packaged in an electronics module can be plugged into a base having a backplane providing connections between circuits in the electronics module and terminals to which the field wiring is connected. The base units are mounted side-by-side on a standard DIN rail and plugged into one another using one-way connectors for communication with a controller over a common bus system. These one way connectors utilize conventional male/female connecting elements which require reciprocal axial movement for engagement and disengagement. In order for modular units to be inserted or removed from a string of modular units without moving adjacent units, one of the connector elements is mounted on a slide which moves axially on the modular unit. In addition to requiring this additional manipulation, the connectors can be only engaged or disengaged when the electronics modules are removed from the base units.
In the system of U.S. Pat. No. 5,472,347, the electronics modules are locked into the base by a cantilevered integral latch on the base. This latch is difficult to access, especially in high density installations where adjacent strings of serially connected modular units are close together so that a tool is required to operate the otherwise manually operable latch.
There is a need, therefore, for an improved I/O apparatus for connecting field wiring with a process control system or a monitoring system.
This includes a need for an improved modular I/O system for such applications.
In particular, there is an need for an improved modular I/O system in which individual modular units can be inserted into or removed from an interconnected string of units without disturbing adjacent units and without the requirement for separate manipulation of connectors.
There is an additional need for an improved modular I/O system which has a more accessible arrangement for locking electronics modules into the bases of the modular units.
There is also a need for an improved modular I/O system having a simple locking arrangement for locking multiple electronics modules in a base of a modular unit.